This invention relates generally to steam turbines and more generally to methods and apparatus for retaining material in hybrid buckets.
Steam turbine buckets (blades) operate in an environment in which they are subject to high centrifugal loads. Additionally, they are in a steam environment with a varying angle of flow incidence to the bucket. A hybrid bucket is a steam turbine bucket that is made primarily of a metallic substance with at least one “pocket” of a non-metallic composite filler material. The filler material may further comprise a polyimide or other type of polymeric resin combined with continuous glass, carbon, KEVLAR® or other fiber reinforcement to achieve the original airfoil surface. This composite matrix is now being designed to be used in units that have high bucket temperatures during windage conditions (low flow, high speed “wind milling” of buckets). One issue with the very stiff high temperature composites is that the adhesion to the metal becomes one of the weakest links in the system.
U.S. Pat. No. 5,720,597, entitled “Multi-Component Blade for Gas Turbine,” describes gas turbine aircraft blades constructed of metal and foam are provided with a composite skin, an erosion coating, or both. Configurations are disclosed that are applicable to fan blades, and more specifically to “propulsion engines.” As such, the sizes and shapes of the pockets are significantly limited. Moreover, U.S. Pat. No. 6,139,728, entitled “Poly-Component Blade for a Steam Turbine,” discloses configurations similar to those disclosed in U.S. Pat. No. 5,720,597, but for steam turbines. Benefits described include lower weight, which allows less robust blade alignment and thereby reduces cost. Furthermore, U.S. Pat. No. 6,042,338, entitled “Detuned Fan Blade Apparatus and Method,” describes a “propulsion engine fan” and various types of blades with different pocket locations, but does not disclose blades of essentially one pocket with different rib structures. In addition, the disclosure is limited to pockets with radial location from a tip to 5%-38% span and chord wise from 15% to 35% from the leading edge and 20% to 45% from the trailing edge with similar limitations on the second or alternative pocket design.